The overall direction of the Molecular Mechanisms of Tumor Promotion Section is to understand the mechanisms underlying the initial events in tumor promotion. Particular emphasis is directed at the factors determining the heterogeneity in response to different members of the phorbol ester class, with special reference to the contribution of protein kinase C isozyme heterogeneity to the heterogeneity of response. The divergent functional role of protein kinase C isozymes has been established both by reconstitution of permeabilized cells and by transfection. In vitro analysis of recombinant, individual isozymes confirms both differences in substrate specificity and in ligand recognition. In intact cells, such differences in isotype-specific ligand recognition are magnified, with differential down-regulation of isotypes by typical phorbol esters compared to the two classes of ligands, the 12- deoxyphorbol 13-monoesters and bryostatins, which function as partial antagonists of protein kinase C. Although protein kinase C is the primary target for the phorbol esters, two other classes of high affinity receptors for these compounds have also been described. We have expressed N-chimaerin in the baculovirus system and are currently exploring its binding properties; similarly, the phorbol ester binding domain of unc-13 is being expressed in a bacterial system and its binding characterized. The highly irritant phorbol-related derivative resiniferatoxin interacts with receptors entirely distinct from those of typical phorbol esters. Its receptors, termed vanilloid receptors, control activation and desensitization of sensory afferent C-fiber neurons, which are involved in nociception, inflammation, and thermoregulation. We have now been able to identify and characterize those receptors in the periphery to compare with those in cell bodies and in the spinal cord. Likewise, we have extended our analysis from model systems to characterize receptors in man.